1. Field of the Invention
This invention relates to differential mechanisms, more particularly to a limited slip differential mechanism having an integral viscous fluid coupling.
2. Description of the Prior Art
During driving, all four wheels of a motor vehicle may not be turning at the same rate of speed. This is commonly encountered when the vehicle is making a turn, but can also be caused by variation in tire sizes, braking and nonuniform road surface conditions. In order to accomodate differing wheel spin rates and yet direct engine power to the two wheels (four wheels in a four wheel drive system), it has been a longstanding solution in the art to provide a differential between the powered wheels. The differential allows the wheels to spin at an independent rate and yet deliver power to the wheels. While the solution has proven satisfactory in most driving conditions, it has proven to be unsatisfactory under conditions where one of the powered wheels experiences a road surface having a much lower coefficient of friction than that experienced by the other powered wheel. In such circumstances, the wheel experiencing the least friction with the road will tend, through the action of the differential, to spin while very little torque is supplied to the wheel experiencing higher friction with the road. This can result under road conditions where typically one of the powered wheels has encountered mud or ice, causing the vehicle to become immobilized.
Several attempts have been made in the prior art to rectify this situation in a manner that modifies the differentiation of the wheels when under the extreme conditions described above. In these solutions, a mechanism is used to cause the shafts of each of the powered wheels to tend to rotate at the same rate of speed. This mechanism may take the form of a direct mechanical coupling, a friction coupling, or a fluidic coupling.
Friction coupling mechanisms are exemplified by U.S. Pat. No. 4,583,424 to von Hiddessen et al which teaches a limited slip differential having a differential bevel gear and pressure discs operated by an axial actuator, and by U.S. Pat. No. 3,987,689 to Engle which teaches a limited slip differential planetary pinion gears and an actuator for applying pressure to friction clutch plates in response to speed differences. Both of these solutions suffer from a tendency toward a jump in differentiation resistance when the clutch plates first engage.
Of the fluidic coupling mechanisms, one class of solution uses impellers moving in a viscous fluid. A representative example is U.S. Pat. No. 3,915,031 to Hanson, which discloses a fillable viscous liquid reservoir in which an impeller, connected to one of the powered wheel shafts, rotates; the viscosity of the liquid causing a resistance to differentiation and a more even distribution of torque between the powered wheels. However, this solution suffers from its reliance upon impellers which move in a high viscous medium, causing engine power to be wasted. U.S. Pat. No. 3,534,633 to Chocholek and U.S. Pat. No. 4,493,227 to Schmid disclose similar teachings.
FIG. 1 shows another class of solution utilizing a viscous liquid 10 contained in a housing 12 having located therein closely spaced annular, apertured plates 14 and 16 each alternately attached to one or the other powered wheel shafts 18 and 20. A representative example is U.S. Pat. No. 2,949,046 to Critelli. The housing 12 contains a planetary and orbit gear differential 22, as well as the above described series of plates 14 and 16. Within the housing is the high viscous fluid 10. In the event one of the wheel shafts turns at a rate of speed different from the other, the plates turn relatively to each other in the liquid, generating a viscous coupling between the plates and, consequently, a tendency for the wheel shafts to turn at the same rate of speed. U.S. Pat. No. 3,760,922 to Rolt et al and U.S. Pat. No. 3,869,940 to Webb et al disclose viscous fluid couplings having interleaved annular plates. U.S. Pat. No. 4,040,271 to Rolt et al and U.S. Pat. No. 4,096,712 to Webb disclose viscous couplings wherein the volume of the chambers is controlled by a spring biased piston.
A serious problem, however, occurs during operation of such viscous fluid and plate couplings. The relative rotation between the plates results in Newtonian fluid flow in the viscous fluid, and the relationship between the shafts and the energy input/output is governed by well known equations of viscous drag. However, frequently the relative rotation between the plates is such as to cause non-Newtonian fluid flow in the viscous fluid and the exact nature of the relationship between the shafts and the input/output energy is not governed by the aforementioned equations.
FIG. 2 shows a typical graph of a response curve 24 for the device shown in FIG. 1. The response curve is a graphical plot of output torque verses time, where the coupling variables are held constant. The coupling variables are defined as the number of plates, their aperture configuration, their dimensioning, their spacing, as well as the viscosity of the fluid and its percentage of fill in the housing.
An analysis of the events taking place which are represented by the graph of FIG. 2, are as follows: Between time T.sub.0 and T.sub.1, energy input to the coupling due to the relative rotation of the plates causes the temperature to rise and the viscosity of fluid to reduce. Hence, the torque across the coupling similarly reduces. Between time T.sub.1 and T.sub.2, pressure and expansion of the fluid due to increasing temperature continues and now overtakes reduction and viscosity as the cause of change of torque across the coupling. Consequently, the torque across the coupling begins to rise. Between time T.sub.2 and T.sub.3 pressure builds rapidly because the fluid is completely expanded to fill the housing. The torque now increases rapidly as well. This is attributable to the interaction of slots and grooves in the plates which may present a "paddle wheel" effect. Between time T.sub.3 and T.sub.4, the limits of the coupling are reached. In some cases the temperature and pressure may continue to rise until the fluid, the coupling, or both are destroyed.
Accordingly, it is not desireable to rely solely upon plate movement within the viscous liquid in order to limit differentiation.
Yet another class of solution has addressed the aforementioned problem by incorporating, besides fluid coupled plates, additional coupling means between the wheel shafts.
One way to accomplish this result is to utilize mutual contact between the plates. Such a solution is disclosed in U.S. Pat. No. 4,022,084 to Pagdin et al. A viscous fluid is used in combination with plates, one set of plates for each wheel shaft, as described above, with the added feature that one of the plate sets is free to move axially. When high relative rotation occurs, the plates tend to come together by an unexpected and undetermined action of the fluid, allowing frictional facings on the plates to make contact.
A second way to accomplish this result is to use the pressure increase developed in the coupling to cause a piston to bring frictional surfaces into contact. U.S. Pat. No. 4,031,780 to Dolan et al and U.S. Pat. No. 4,048,872 to Webb disclose a friction clutch activated by volume expansion of the viscous fluid due to a pressure increase caused by the shearing action of the plates contained therein.
Particularly relevant to the present invention is U.S. Pat. No. 4,058,027 to Webb, a variation on this approach, where two separate couplings of interleaved plates are used in which the plates of one of the couplings are forced into mutual frictional contact by action of expansion of the viscous fluid in the other coupling.
The aforementioned class of the viscous fluid type solution suffers from the drawback that there is a tendency towards a quantum jump in torque output when the clutch elements engage, rather than a continuous increase in torque output.
Finally, U.S. Pat. No. 3,924,489 to Yasuda and U.S. Pat. No. 4,458,559 to Croswhite et al disclose turbine systems which allow for variable torque transfer. The turbine mechanisms in these devices are complicated, causing their use to be limited.
Accordingly, there remains in the art the need to provide a differential having a viscous coupling which functions as a limited slip differential in a motor vehicle, particularly front wheel drive automobiles, in which initial torque output is small and thereafter the torque output smoothly increases until a maximum safe operating torque output is reached.